Preheat the oven to 175°C (350°F) with a rack in the centre. Grease a 23 x 13 cm (9 x 5 inch) loaf tin and line with a strip of parchment that overhangs the long sides.
Mash the very ripe bananas in a large bowl until almost smooth.
Whisk in the eggs, Greek yogurt, maple syrup and oil.
Add the oat flour, protein powder, baking powder and cinnamon and fold until just combined. Fold in the chopped chocolate.
Scrape the batter into the tin and smooth the top with a spatula.
Bake for 45 to 50 minutes, until a skewer in the centre comes out clean. Cover loosely with foil after 30 minutes if it browns too fast.
Cool in the tin for 15 minutes, then lift out and cool completely on a rack before slicing into 10 pieces.